"The Context of XSL Transformations and the XML Path Language"
From the book: http://www.xml.com/lpt/a/2000/08/holman/s1.html#purcxsl
More examples: http://www.xml.com/lpt/a/2000/08/holman/s2_1.html
This is a pretty good overview of how XML, valid XML, XPath, XSL, and XSLT work.
--Wim